1. Technical Field
The present invention relates to a photocurable ink composition set, and an ink jet recording method and recorded matter using the same.
2. Related Art
Photocurable ink compositions are increasingly being used for manufacture of color filters, printing on print boards and plastic materials such as plastic cards, plastic sheets, and plastic components, printing for large sign boards and outdoor and indoor advertisements, and printing of bar codes and dates.
Japanese Unexamined Patent Application Publication No. 2004-99796 discloses a photocurable ink composition containing a dendrimer, which is a type of dendritic polymer. Dendritic polymers have a molecular structure in which functional groups are more closely present at the surface than general linear polymers, and are therefore expected as functional nano-polymer materials.
Unfortunately, the addition of a dendritic polymer to an ink composition significantly increases the viscosity of the ink. The increase of the viscosity may cause the ink jet head to fail the ejection of the ink. In order to prevent the increase of the viscosity of the ink composition, the dendritic polymer content may be reduce. However, this cancels the effect of the dendritic polymer to increase the characteristics of the ink composition.
In order to prevent the increase of the viscosity of an ink, a low-viscosity polymerizable compound is generally used as a diluent monomer. However, low-viscosity monomers generally have low molecular weights, and often have low flash points and odors and cause skin irritation, thus having health and safety issues. Alternatively, water or an organic solvent having no polymerizable functional group may be added as a low-viscosity diluent. The addition of such a constituent not involved in curing reaction, however, undesirably requires additional, special treatment and increases the load of the manufacturing process. For example, a drying step may be performed by heating or blowing before curing, or an absorption layer may be formed on the print medium. If curing reaction is conducted before the additive is completely dried, air bubbles may be generated from the residual solvent or water by heat for polymerization, or the residual solvent or water may bleed from the cured composition to remain at the surface and cause tackiness.
In addition, many photocurable ink compositions are unstable in storage, and their viscosities are increased to gelate. This is because the ink composition is itself reactive, and is intrinsically disadvantageous. It is self-evident that as the curing reactivity is increased, the storage stability is degraded. Accordingly, a large amount of thermal polymerization inhibiter inhibiting dark reactions is added to the ink composition for storage stability. This however inhibits the polymerization for curing the ink and makes it difficult to form appropriate images. In particular, it is important to enhance much the storage stability of the dendrimer or hyperbranched polymer, which has a molecular structure in which polymerizable functional groups are closely and convergently disposed at the outermost surface. For use of a dendritic polymer in an ink composition, a two-part ink composition set including two compositions is effective which are stored in two parts and mixed before use.